In answer to your first point. Any reclearance replaces the previous one. End of story. That is what our rule book says.
Inbound LAM, what should technically happen is that you should be told to "descend to FLxxx to be FL150 or below by SABRE." As I'm sure you are aware this rarely happens due time constraints and R/T congestion.
The problem is that most controllers will expect you to still make the initial level restriction as my colleague above mentioned. If it were a level restriction used to guarantee seperation,personally I would be extremely careful with any reclearances.